Connector Corrosion Protection

Connectors are the most vulnerable
points in a circuit. Corrosion, from either oxidation or galvanization, reduces
current-carrying capacity and results in intermittent, and ultimately
permanent, failure of the circuit. In harsh environments, the major cause of
connector failure is galvanic corrosion, a process in which dissimilar metals
give up or collect electrons in the presence of an electrolyte, usually water.

Although many connectors are designed
for use in harsh environments, too often their service lives are limited by
corrosion due to gaps and other leak paths (even microscopic porosity) in the
wires, insulation, plastic housing and pins. Prior to complete failure, a
corroding connector may cause its circuit to fail intermittently, causing
downtime and maintenance expense while frustrated mechanics search for the
source of the problem.

Temperature cycling causes the
dissimilar materials in connectors to expand and contract at different rates. Exacerbated
by this and other stressful conditions, such as vibration or repeated bending,
leak paths allow penetration by the main culprit - moisture. In some cases,
leak paths even function as wicks drawing in moisture.

Harsh environments where connectors are
commonly used include automobile engine compartments; military, aircraft and aerospace
equipment; outdoor devices; and a range of industrial facilities. Lubricants
and coolants used to keep automated assembly lines running can attack plastic
insulating materials, as can the steam and caustic chemicals regularly used to
wash down certain food-processing equipment. And in marine applications such as
shipping and offshore oil rigs, corrosion is further accelerated by sea salt.

With
these applications in mind, there are two basic ways to minimize corrosion to
consider when designing a system and specifying the connectors that will be
used in it: plating and sealing.

For
many years, plating of an electronic connector's contacts with a layer of tin
has been a common practice, especially where copper and aluminum are both
present. This not only reduces electron transfer, but also lowers resistance
and prevents the discoloration of bare copper. Aluminum contacts, if not
plated, are often coated with an oxide-inhibiting compound. However, neither
practice is effective against harsh environments in the long run.

Sealing,
if done properly, closes off the leak paths that moisture and oxygen can follow.
Sometimes, sealing is done in addition to plating. Silicone-based sealants are
effective in many applications, as are epoxy-based potting compounds. Both
types are typically applied by hand to the connectors, but this is a relatively
expensive solution to the corrosion problem. In some cases, these hand-applied
sealants may make final assembly of the end product more difficult and
time-consuming, since they are usually not uniformly applied. Another factor
affecting sealed connector is that, in some automated assembly machines,
connectors that don't slide together easily can shut down the whole assembly
line.

New Resin OptionA
recent advance in connector sealants, methacrylate polymer resin, provides
maximum protection for connectors in harsh environments. Instead of being
painted or brushed onto individual connectors one by one, these nonconductive
resins can uniformly and economically seal a large batch of connectors,
assemblies, and wiring harnesses all at once, in a process called vacuum
impregnation.

Vacuum
impregnation technology has been employed for decades in the complete sealing
of leaks resulting from porosity in metal castings and powdered-metal parts. In
liquid form, the resins-either thermoset or anaerobic-are forced into all voids
in a casting, usually via vacuum and pressure. The material is then washed and
cured, completely sealing porosity and leaving the part leak-free, even under
pressure.

There
are four common methods of impregnation: dry vacuum and pressure, internal pressure,
wet vacuum and pressure and wet vacuum only. The leak paths and porosity in
connector components are easily filled with the wet vacuum only approach. Inside
a vacuum chamber, connectors, assemblies, or entire wiring harnesses are placed
in a bath of low-viscosity impregnating resin and air is entirely evacuated
from the chamber. All air in the connectors, wires, etc. rises to the surface
of the bath and is removed from the chamber. Once the vacuum is drawn, the
return to normal atmospheric pressure is enough to drive the resin into all
leak paths.

At
this point, the product is water-washed prior to curing, to prevent the sealant
from interfering with conductivity. Next, the resin is cured in a hot bath -
usually at about 140F. Though not necessary for the curing process, ions from
copper, aluminum, or iron inside connectors can actually function as catalysts,
assisting in the curing process. These metals give up electrons to the resin as
if it were a more cathodic metal. Anaerobic resin has proven particularly
effective for sealing connectors because it does not require air to cure.

Once
cured, the resin is irreversibly cross-linked and will not reliquify. It will
withstand temperatures up to 350F and resist solvents, Freon, steam, oil,
gasoline, glycols and printing inks.

Yet
another benefit of these resins is their potential for flexibility. When
necessary for the anticipated application, the resin can be formulated to cure
to a flexible state.

Until
recently, there has been a drawback to most impregnated thermoset resins - they
tend to ignite when exposed to fire. Fortunately, fire-resistant resins are now
on the market.

After
the impregnation process, a simple air-pressure test can be performed to prove
the connector assembly is thoroughly sealed against ambient moisture and salts
that could otherwise cause corrosion and product failure.

Extreme Environment
Application

The
durability of connectors sealed in this manner can be especially helpful in
installations with prolonged conditions of thermal cycling, vibration and
moisture changes, such as those encountered in many military/aerospace
applications.

In
tanks and other land vehicles, vibration and frequent temperature changes call
for rugged, tightly sealed connections. Humidity, rain and washdowns supply
plenty of moisture, which is of course more abundant for ships, boats and
amphibious vehicles.

The
U.S. Air Force Research Laboratory has concluded that corrosion at the
junctions of connectors carrying signals is one of the most frequent causes of
intermittent failure and other erratic performance in aircraft electrical
systems.

High-altitude
pressure drops can draw trapped air out of a connector, possibly opening new
pathways for corrosion-causing moisture. Moisture can then quickly enter such
pathways in wet weather or during washdown/de-icing operations. Condensation or
water vapor in humid air could be forced into the pathways as the aircraft
descends and air pressure rises. A connector totally sealed using vacuum
impregnation has no air pockets, since they would be filled with resin.

Created crazy problems... the whiskers are soooo thin, (but highly conductive) they are hard to see without magnification. easy to break, but grow back. Nothing but intermittent problems for the circuitry involved.

Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.